Projection system

ABSTRACT

A projection system for image display with at least one lamp ( 1 ), with at least one sensor ( 5 ) for detecting changes in the luminous flux delivered by said at least one lamp ( 1 ) and for compensating these changes through a suitable control of the image display and/or the lamp is described. The projection system is remarkable in that a light integrator ( 3 ) is provided, into which at least a portion of the light provided by the lamp ( 1 ) is coupled in, while the sensor ( 5 ) is optically coupled to the light integrator ( 3 ) such that it detects the luminous intensity present in the light integrator ( 3 ). Since this luminous intensity is very homogeneous because of the multiple reflections and is not influenced by brightness fluctuations caused by an optical component such as, for example, a color modulator ( 4 ), a very accurate compensation of changes in the luminous flux generated by the lamp ( 1 ) is made possible by the sensor signal.

The invention relates to a projection system for image display with atleast one lamp and at least one sensor for detecting changes in theluminous flux delivered by said at least one lamp and for compensatingthese changes through a suitable control of the image display and/or thelamp.

One or several high-pressure gas discharge lamps (HID [high intensitydischarge] lamps or UHP [ultra high performance] lamps) are generallyused as light sources in projection systems. An advantage of these lampsis inter alia that they have a comparatively short discharge arc andthus a very small dimension of the luminous surface, so that a very highproportion of the generated light can be directed into an imagingsystem, for example by means of a reflector in whose focus the dischargearc is located. The advantages of the almost point-shaped light emissionmay be utilized correspondingly also for other applications such as, forexample, in spotlights or for illumination purposes, because theradiation characteristic of a reflector can be approximatedsubstantially more closely to a desired ideal gradient thereby.

The small luminous region, however, also involves the risk that thesystem is defocused in the case of only a small localized shift betweenreflector and lamp or discharge arc, whereby the radiationcharacteristic and thus the luminous flux in given locations isconsiderably changed. These shifts may be caused in particular by a leapof the discharge arc, for example owing to an erosion of the electrodesand the concomitant change in their shape or their state.

This may lead to interfering fluctuations in the brightness of thegenerated image which are perceived as unpleasant, in particular in thecase of an imaging system, because the proportion of the light coupledinto the imaging system changes correspondingly.

The lamps mentioned above may be operated in principle both with directcurrent and with alternating current. Both modes of operation have theiradvantages and disadvantages. A quick erosion of the electrodes isprevented and the luminous efficacy of the lamp can be increased by analternating current, but the arc discharge is often unstable here owingto the polarity change, so that periodic brightness fluctuations orother image disturbances may arise. With direct current, however, itcannot be excluded either that instabilities of the arc discharge arise,in particular with an increasing duration of operation, for exampleowing to an electrode spacing that has become irregular in theintervening time, which may manifest itself in particular in the form ofarc leaps.

To safeguard an optimum, interference-free image quality throughout thelife of a discharge lamp, therefore, sensors should be provided in bothmodes of operation for monitoring the luminous flux provided and forproviding a suitable compensation of short-term fluctuations (andpossibly also a long-term luminous decrement).

Fluctuations in the emitted luminous flux may become particularlyunpleasant, in particular in color projection systems operating withtime-sequential color rendering methods, if one of the primary colors isshown with a brightness different from that of the other primary colors,or if the brightness of this one color in certain image regions of thedisplay differs from the brightness in other regions of the display.

Two time-sequential color display methods are distinguished and utilizedin particular nowadays:

In a first method, the color image is generated on the display through asequential representation of full pictures in the three basic colors(“field sequential color”) plus possibly a fourth, white image. Thismethod is used at the moment, for example, in most DLP (digital lightprocessing) projectors.

In a second method, the color image is generated in that the primarycolors run over the display one after the other in the form of colorbeams or color strips (“scrolling color”). This method is used, forexample, by the present applicant's LCOS (liquid crystal on silicon)displays (cf. Shimizu: “Scrolling Color LCOS for HDTV Rear Projection”,in SID 01 Digest of Technical Papers vol. XXXII, pp. 1072 to 1075,2001), and SCR-DMD (sequential color recapture-digital micro mirror)projection displays (cf. Dewald, Penn, Davis: “Sequential ColorRecapture and Dynamic Filtering: A Method of Scrolling Color” in SID 01Digest of Technical Papers, vol. XXXII, pp. 1076 to 1079, 2001).

These systems comprise a color separation or color filtering and amodulator for the color components between the lamp and the display forthe generation of light with the three primary colors. The colorseparation and the modulator may be integrated with one another to agreater or lesser extent. Thus the color filtering and modulation arecarried out by a rotating filter wheel in the SCR systems, whereas thecolor filtering takes place with mirrors and the modulation with prismsin the LCOS system of the present applicant. It is common to allsystems, however, that the modulation causes considerable brightnessfluctuations in the optical system. Furthermore, the sensitivity ofconventional sensors to the various color components is very different.The fluctuations thus caused in the output signal of a sensor arrangedin the radiation path or at the display render this sensor useless forcontrolling the lamp or the image brightness.

In addition, the sensor is to sense a signal which is exactlyproportional to the luminous flux actually hitting the display if it isto render possible a correct control. This is usually not guaranteed,not for positions of the sensor outside the radiation path of the light,and not for positions in front of the optical integration either.

DE 101 36 474.1, for example, discloses an electronic control circuitfor operating a HID or UHP lamp, comprising a lamp driver for providinga controlled lamp current for the lamp and a brightness sensor forgenerating a sensor signal representing the luminous flux generated bythe lamp. A high-pass or bandpass filter is furthermore provided, bymeans of which the sensor signal is filtered and is subsequentlysupplied to the lamp driver for controlling the lamp current.

The object of the high-pass or bandpass filter is to separate long-termchanges in the luminous flux provided by the lamp, in particular aluminous decrement as lamp life progresses, from the short-termfluctuations caused by arc leaps, such that only the latter fluctuationsare used in the active control of the lamp power by the lamp driver.

Such an active control (LOC-light output control), however, cannotoperate reliably if the sensor signal is superimposed with interferingcomponents which are caused, for example, by the brightness fluctuationsoriginating from a color modulator, as was explained above.

It is accordingly an object of the invention to provide a projectionsystem of the kind mentioned in the opening paragraph in whichimpairments of the image quality caused by unwanted changes in theluminous flux provided by the lamp (in particular owing to arc leaps)can be avoided at least substantially also in the presence of regularbrightness fluctuations caused by an optical component of the projectionsystem.

The invention particularly aims to provide a projection system whichcomprises at least one high-pressure gas discharge lamp and in whichimpairments of the image quality owing to fluctuations in the generatedluminous flux, in particular caused by an unstable arc discharge, can beavoided at least substantially also with the use of a time-sequentialcolor representation.

Finally, the invention also aims to provide a projection system withtime-sequential color representation in which color artifacts caused byan unwanted change in the luminous flux provided by the lamp are avoidedat least substantially, in particular if one or several high-pressuregas discharge lamps operated on alternating current are used as the lampor lamps.

The object is achieved according to claim 1 by means of a projectionsystem for image display with at least one lamp and at least one sensorfor detecting changes in the luminous flux delivered by said at leastone lamp and for compensating these changes through a suitable controlof the image display and/or the lamp, and with a light integrator intowhich at least a portion of the light provided by the lamp is coupledin, wherein the sensor is optically coupled to the light integrator suchthat it detects the luminous intensity present in the light integrator.

Since the light entering the light integrator including the lightcomponents possibly reflected back by a color modulator into the exitsurface of the light integrator is homogenized by multiple reflections,the generated sensor signal is at least not substantially superimposedwith interfering components of the color modulator or other opticalcomponents in the projection system, so that it can be used forcontrolling the image display and/or the lamp. A suitable dimensioningof the length of the light integrator renders it possible to reduce theinterfering components to an acceptable level, or indeed substantiallyto any extent desired.

A particular advantage of this solution is that such a light integratoris usually already present in the color projection systems mentioned inthe opening paragraph, so that no measures are necessary in the directlight path and the projection system according to the invention can berealized with a comparatively small additional expenditure.

Furthermore, the sensor is not positioned in the light path of theprojection system and thus causes no perceivable interferences or shadoweffects, i.e. light losses.

Finally, the sensor signal generated in accordance with the inventionmay also be used for the active control of the lamp (LOC) mentionedabove.

The dependent claims relate to advantageous further embodiments of theinvention.

The embodiments of claims 2, 3, and 4 relate to preferred methods ofoptically coupling the at least one sensor to the light integrator.

A suitable arrangement or positioning of the at least one sensor incertain regions or locations of the light integrator as claimed in claim5 and/or 6 renders it possible to optimize the detection of the luminousintensity in particular in those cases in which colored light componentsare reflected back into the light integrator through an exit surfacethereof, for example by a color modulator.

Claim 7 relates to a preferred control of the image representation forthe purpose of compensating changes in the luminous flux provided by thelamp.

A filtering of the sensor signal according to claim 8 renders itpossible to make a purpose-oriented choice of the changes in theluminous flux that are to be compensated in relation to their frequency.

Claim 9, finally, relates to a preferred application of the principle ofthe invention.

Further details, features, and advantages of the invention will becomeapparent from the ensuing description of embodiments which are shown byway of example in the drawing, in which:

FIG. 1 diagrammatically shows essential components of an SCR projectionsystem with a first sensor positioning;

FIG. 2 shows a detail from FIG. 1 with a second sensor positioning; and

FIG. 3 shows a detail from FIG. 1 with a third sensor positioning.

The invention will now be explained with reference to a projectionsystem operating by the second method mentioned above (scrolling colorsystem) with an SCR-DMD display. The construction and manner ofoperation of such a projection system are explained in detail in thecited article by Dewald, Penn, Davis: “Sequential Color Recapture andDynamic Filtering: A Method of Scrolling Color” in SID 01 Digest ofTechnical Papers, vol. XXXII, pp. 1076 to 1079, 2001. This article is tobe considered included in the present description by reference.

FIG. 1 shows the construction principle of the lighting portion of sucha projection system. This Figure shows a light source with at least onelamp 1 and at least one reflector 2 as well as a light integrator (rodintegrator) 3, into whose entry window 31 the light generated by thelamp 1 is focused in the form of a light cone L formed by the reflector2. The light integrator 3 has an exit surface 32 at an end opposed tothe entry window 31, at which surface 32 a color wheel 4 is arranged.

The lamp 1 is in particular a high-pressure gas discharge lamp (HID[high intensity discharge] lamp or UHP [ultra high performance] lamp).

The light integrator 3 (provided it is long enough) generates ahomogeneously distributed luminous intensity locally and temporally atits exit surface 32. The light integrator 3 for this purpose comprises ahighly reflective sheath 33 which encloses a hollow space 34. The lightcoupled into the entry window 31 is multiply reflected against thesheath 33, as are the light components reflected back by reflection atthe color wheel 4 through the exit surface 32 into the light integrator3, and the light is homogenized, given a sufficient length of the lightintegrator 3, such that the desired, homogeneous distribution ofluminous intensity is achieved at the exit surface 32 thereof. The entrywindow 31 is made as small as possible in order to minimize light lossescaused thereby.

The light integrator 3 may alternatively be formed by a solid opticalwaveguide of an optically guiding material, in particular glass or asuitable synthetic resin.

The color wheel 4 which is known per se is arranged at the exit surface32. This color wheel 4 (color modulator) comprises red, green, blue, andtransparent coatings, all diachronically reflecting, which are arrangedin the form of an RGB pattern of Archimedean spirals. The pattern isdimensioned such that at any time one or several colored spirals coverthe cross-section of the exit surface 32 of the light integrator 10. Thepattern has the characteristic that the boundaries between the colorsred, green, and blue move with constant velocity in radial directionwhen the color wheel 4 is rotated. As a result, the RGB pattern of thecolor wheel 4 moves with substantially constant velocity over the exitsurface 32 of the light integrator 3. The distance between the exitsurface 32 and the color wheel 4 should be as small as possible so as toavoid light losses.

The RGB pattern generated by the color wheel 4 is directed at a DMDdisplay by means of a relay lens (projection optics), both componentsnot being shown, which display is controlled in a known manner by acontrol device. Rotation of the color wheel 4 creates the color stripssequentially traversing the DMD display, as described above. The imagegenerated on the DMD display is finally projected onto a wall or ascreen or some similar item (not shown) by means of a lens.

At least one sensor 5 is provided, which is connected to a lamp driver(power supply unit) 6 of the lamp for the purpose of avoiding brightnessfluctuations in the image caused by changes in the luminous flux of thelamp, for example owing to leaps of the discharge arc in the lamp 1,caused again by an unwanted change in the lamp current or other effects,which sensor 5 controls the lamp on the basis of the detected luminousintensity such that the lamp current is increased when the luminous fluxdecreases and is decreased when the luminous flux increases.

The sensor 5 is optically coupled to the light integrator 3 such thatthe sensor detects the luminous intensity inside the light integrator 3.The light here is very homogeneous, as was explained above, and is notsubject to the brightness fluctuations caused by the color wheel 4.Changes in the luminous flux generated by the lamp 1 can thus bedetected free from interferences and can be effectively compensated bymeans of a suitable control of the lamp driver 6.

The sensor 5 is preferably arranged such that it detects exclusively thelight present in the light integrator 3. This may be achieved in thatthe sensor 5 is directly mounted against the sheath 33, as in FIG. 1,which sheath is provided with an at least partly transmitting window forthe sensor 5.

Furthermore, the sensor 5 may also be optically connected to the hollowspace 34 of the light integrator 3 via an optical waveguide, or it mayeven itself be arranged inside the hollow space 34 of the lightintegrator 3, provided it is sufficiently temperature-resistant.

FIGS. 2 and 3 show portions from FIG. 1 on an enlarged scale. The lightintegrator 3 with its sheath 33 and the hollow space 34 is shown indetail here. A light cone L of a light source is directed again into theentry window 31, while at the opposite end of the light integrator 3 acolor modulator is present which generates the diagrammaticallyindicated primary colors red (R), green (G), and blue (B). The colormodulator reflects light components LR of these primary colors back intothe light integrator 3 through the exit surface 32 thereof.

It is to be taken into account in the choice of an optimized sensorposition that the color strips move one after the other over the exitsurface 32 of the light integrator 3 and that the light components LRreflected back may possibly not be optimally mixed with the light Lcoupled into the entry window 31 in the case of a too short lightintegrator 3 because of the small number of reflections. In this casethe sensor signal will fluctuate in the frequency of the color strips.

To avoid this, a sensor position is to be chosen which is as evenlyexposed as possible to all reflections. This means that rays of allcolor components should hit the sensor in as equal a measure aspossible, also if these rays traverse the exit surface 32 of the lightintegrator 3 in conformity with the movement of the color strips.

FIG. 2 shows by way of example a first positioning in which for thispurpose a sensor surface in the form of a light-receiving strip 51 (forexample made of glass or synthetic resin) is provided on the sheath 33of the light integrator 3, such that the strip 51 extends substantiallyparallel to the exit surface 32 of the light integrator 3, and thesheath 33 is at least partly transmittive to the light present in thelight integrator 3 below the strip 51. The strip 51 may extend over thefull circumference of the light integrator 3 or only over a portion ofits circumference, or its height and/or width. The width of the strip 51preferably corresponds to approximately one color cycle here.

The strip 51 in this position receives the light components LR reflectedback substantially directly, i.e. without previous reflection againstthe sheath 33 of the light integrator 3.

The sensor 5 proper may be arranged in a position along this strip 51and may be, for example, a known semiconductor sensor, or the strip 51itself is constructed, for example, as a (silicon) sensor.

FIG. 3 shows a second positioning in which the light-receiving strip 51provided on the sheath 33 extends substantially perpendicularly to theexit surface 32, i.e. in axial direction of the light integrator 3 alongat least a portion of the length thereof. Below the strip 51, the sheath33 is again partly transmittive to the light present in the lightintegrator 3. The width of the strip 51 is determined substantially independence on the color filters and the angles of the rays LR reflectedagainst the sheath 33.

Given this positioning, the strip 51 accepts the light components LRreflected back substantially after a reflection against the sheath 33 ofthe light integrator 3.

The sensor 5 proper may be arranged in a location along the strip 51also in this case and may be, for example, a known semiconductor sensor,or the strip 51 itself is constructed, for example, as a (silicon)sensor.

The use of the light-receiving strip 51 improves the coupling-out oflight owing to a better mixing of all light components in both cases.

It is true in general that the sensor arrangement and positioning arethe more uncritical as the light integrator 3 is longer.

An advantage of the sensor arrangements is also that the local andtemporal homogeneity of the luminous intensity at the exit surface 32 ofthe light integrator 3 can be improved thereby also during the timeintervals in which the lamp provides a constant luminous flux. Anoverall improvement in the image quality is obtained in this manner.

The principle of the invention may be advantageously combined also withthe electronic circuit for operating a HID or UHP lamp known from thecited DE 101 36 474.1 when the brightness sensor described therein isreplaced with a sensor arranged in the manner of the present invention.

In the embodiments described above, the control of the image display,whereby changes in the luminous flux generated by the lamp arecompensated, takes place through a control of the lamp current (and thusof the image brightness) in that the sensor signal is applied to thelamp driver 6.

Alternatively or in addition thereto, however, it is also possible tochange the brightness of the image by means of an optical filter thatcan be electrically controlled by the sensor signal and that isintroduced (additionally) into the radiation path between the lamp andthe display, and/or by means of a gray level mask in the form of afactor with which the brightness of the image representation on thedisplay is influenced in dependence on the sensor signal.

These two alternative brightness controls, which are particularlysuitable for the very fast displays used in the DLP systems, aredescribed in detail in DE 102 20 510.8. This publication is to beregarded as forming part of the present disclosure by reference, so thatit need not be discussed in any detail below.

The principle of the invention is obviously also applicable to thoselighting systems which in themselves do not comprise a light integrator,to the extent to which the application and the construction of such asystem render possible the use of a corresponding light integrator in atleast a portion of the light path.

1. A projection system for image display with at least one lamp (1),with at least one sensor (5) for detecting changes in the luminous fluxdelivered by said at least one lamp (1) and for compensating thesechanges through a suitable control of the image display and/or the lamp,and with a light integrator (3) into which at least a portion of thelight provided by the lamp (1) is coupled in, wherein the sensor (5) isoptically coupled to the light integrator (3) such that it detects theluminous intensity present in the light integrator (3).
 2. A projectionsystem as claimed in claim 1, wherein the sensor (5) is arranged at asheath (33) of the light integrator (3), said sheath (33) having awindow which is at least partly transmittive to the light present in thelight integrator (3), through which window the light is incident on thesensor (5).
 3. A projection system as claimed in claim 1, wherein the atleast one sensor (5) is arranged inside the light integrator (3).
 4. Aprojection system as claimed in claim 1, wherein the at least one sensor(5) is optically coupled to the light integrator (3) by means of anoptical waveguide.
 5. A projection system as claimed in claim 1, whereinthe at least one sensor (5) has a sensor surface (51) against or insidethe light integrator (3), which surface extends substantially parallelto the exit surface (32) of the light integrator (3).
 6. A projectionsystem as claimed in claim 1, wherein the at least one sensor (5) has asensor surface (51) against or inside the light integrator (3), whichsurface extends substantially perpendicularly to the exit surface (32)of the light integrator (3).
 7. A projection system as claimed in claim1, wherein the control of the image representation takes place in thatthe output signal of the at least one sensor (5) is applied to a lampdriver (6).
 8. A projection system as claimed in claim 1, wherein theoutput signal of the at least one sensor (5) is filtered by means of afilter for the purpose of compensating changes in the luminous fluxgenerated by the lamp (1) which occur with a given frequency.
 9. Aprojection system as claimed in claim 1 for the representation of colorimages through the time-sequential generation of primary colors on adisplay, comprising a light integrator (3) to which the at least onesensor (5) is optically coupled.